<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>边框绘制三角形</title>
  </head>
  <style>
    .box {
      width: 0px;
      height: 0px;
      border: 50px solid transparent;
      border-bottom: 50px solid red;
    }
  </style>
  <body>
    <div class="box"></div>
  </body>
</html>
